1. Field of the Invention
The present invention relates to a system and method for equalizing and optimizing the transmission characteristics in wavelength division multiplexing optical communication systems.
2. Description of the Related Art
Many business companies are now beginning to adopt wavelength division multiplexing (WDM) as the core technology for high-speed networks. Wave division multiplexing is a technique which combines a plurality of carrier waves of different wavelengths, each modulated with a data signal, for simultaneous transmission over a single-core optical fiber cable and utilizes the property that optical beams of different wavelengths do not interfere with one another.
In optical communication systems using such WDM technology, the transmission characteristics are observed in terms of optical signal to noise ratios (OSNR: optical SN ratio) and Q factors.
Here, we explain the background of the introduction of the Q factors as a new evaluation index for the transmission characteristics. In recent years, international traffic has kept increasing with the increase in the capacity of communication networks typified by the Internet. With the rapid increase in traffic, optical submarine cables are actively being installed all over the world. Typical example of submarine cables are Pacific-Ocean submarine cables and Atlantic-Ocean submarine cables.
Recent optical submarine cable systems, unlike former systems, use erbium-doped fiber amplifiers (EDFA) to realize high-capacity long-haul communication with all-light relay and without any optoelectronic conversion. The EDFA-based relay system involves no retiming operation and requires only non-linear amplification for amplifying directly optical signals; thus, there is little to limit the communication speed, the degree of freedom can be increased, and the speed can be increased relatively easily. On the other hand, since no retiming operation is involved, the noise produced at each stage cannot be removed, which affects succeeding stages. For this reason, it is difficult to evaluate the reliability of optical communication systems based on optical amplification using EDFAs.
There is a slight increase in noise in each of the optical fiber amplifiers. Merely measuring the BER (Bit Error Rate), as was done before, is not sufficient to determine the degree of the effect. Therefore, measurement of the Q factor was proposed.
The Q factor is the signal to noise ratio in a digital signal, assuming that noise signals are Gaussian noise and represented by the signal amplitude (μ) of the digital signal, and the effective Q factor (σ) of noise amplitude is as follows:Q=Abs(μ1−μ0)/(σ1+σ0)  (1)where Abs (μ1−μ0) is the absolute value of μ1−μ0.
In the wavelength division multiplexing optical communication system, it is desirable that the transmission characteristics be equalized for all channels. In real systems, however, degradation of the transmission characteristics varies from channel to channel. Thus, even if transmissions are made from the transmitting station using the same power for all channels, the power received by the receiving station varies from channel to channel and the same transmission characteristic is not maintained for all channels. In addition, the transmission characteristics vary with time as well. For this reason, it is required to perform pre-emphasis processing in the transmitting station. Pre-emphasis processing is an operation in the transmitting station of controlling the power of a optical signal to be transmitted for each channel so that, in the receiving station, the signal to noise ratio (SNR) becomes the same for all channels.
In prior art techniques, equalizing the transmission characteristics depends greatly on the pre-emphasis processing in the terminal station. However, the amounts of pre-emphasis have an upper limiting value (the difference between the maximum optical signal power and the minimum optical signal power allowed for pre-emphasis). When the upper limiting value is exceeded, there arises the possibility of degradation in the transmission characteristics due to a non-linear effect and degradation in OSNR in the terminal station. Thus, in order to ensure that the system transmission characteristics above are a fixed criterion, it is required to set up very stringent criteria in the design of repeaters and the like.
Particularly, as in submarine optical fiber communication systems, and in systems expected to experience increases in transmission distance and speed in the future, it is anticipated that limitations in the amounts of pre-emphasis and design criteria of repeaters will become increasingly strict and mere conventional pre-emphasis based transmission characteristic equalization control will not be able to achieve the performance required of the systems
In each repeater, the gain is equalized by EDF (erbium-doped fiber) and LPG (long-period grating). However, the gain cannot be equalized perfectly due to manufacturing errors of EDF and LPG. It is therefore required to equalize the gain by subjecting the transmission factor versus wavelength characteristic to variable control every ten repeaters. However, such variable control has heretofore not been performed; instead, a selection is made from a plurality of previously prepared optical filters, each with a fixed transmission factor versus wavelength characteristic. With such a system, however, it is not impossible to equalize the gain with flexibility.
Recently, it has been shown that the equalization of transmission characteristics using variable filters and pre-emphasis in combination provides transmission characteristics of better quality than the equalization of transmission characteristics based on pre-emphasis only (see T. Naito et al., “Active Gain Slope Compensation in Large-Capacity, Long-Haul WDM transmission System”, 10-th Optical Amplifiers and Their Applications, WC5-3, 1999).